58 TERRESTRIAL ADAPTATIONS.

of organic nature, with the view of fixing the
readers attention upon the correspondence of the
two.

1. The succession and alternation, at any given
place, of heat and cold, rain and sunshine, wind
and calm, and other atmospheric changes, appears
at first sight to be extremely irregular, and not
subject to any law. It is, however, easy to see,
with a little attention, that there is a certain de-
gree of constancy in the average weather and
seasons of each place, though the particular facts
of which these generalities are made up seem to
be out of the reach of fixed laws. And when we
apply any numerical measure to these particular
occurrences, and take the average of the numbers
thus observed, we generally find a remarkably
close correspondence in the numbers belonging to
the whole, or to analogous portions of successive
years. This will be found to apply to the mea—
sures given by the thermometer, the barometer,
the hygrometer, the raingage, and similar instru-
ments. Thus it is found that very hot summers,
or very cold xvinters, raise or depress the mean
annual temperature very little above or below the
general standard.

The heat may be expressed by degrees of the
thermometer; the temperature of the day is esti-
mated by this measure taken at a certain period
of the day, which is found by experience to cor-
respond with the daily average; and the mean